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Trig help please

  1. Aug 20, 2007 #1
    Question 1

    sin theta = 2/3 tan theta < 0

    sin2 theta = ??

    how would i do this?
    please help

    Question 2


    2cos^2 theta - cos theta = 1 for 0 <= theta < 2pie

    how would i do this one too?
  2. jcsd
  3. Aug 20, 2007 #2


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    Science Advisor
    Homework Helper

    The rules of the forum are that you have to at least try the problem before we can really help. But I can help a little, on the first one square both sides and remember, tan(theta)=sin(theta)/cos(theta) and cos^2(theta)=1-sin^2(theta).
    Last edited: Aug 20, 2007
  4. Aug 20, 2007 #3


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    For the first question, you ought to know that [tex] tan \theta = \frac{sin\theta}{cos \theta} [/tex] and that [tex]sin2\theta = 2 sin\theta cos\theta[/tex]. The second question is just a quadratic equation.
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